The influence of catalyst pre-treatment temperature (650 and 750 degrees C) and oxygen concentration (lambda = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co3O4/CeO2 and Co3O4/CeO2-ZrO2 containing 30 wt.% of Co3O4. The catalytic materials prepared by the co-precipitation method were calcined at 650 degrees C for 5 h (fresh samples); a portion of them was further treated at 750 degrees C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g(-1) h(-1)). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2-ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2-ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750-800 degrees C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 degrees C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess lambda = 8, stoichiometric lambda = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria-zirconia against Co3O4 decomposition into Coo was observed. Moreover, the role of ceria and ceria-zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. (c) 2006 Elsevier B.V. All rights reserved.